This invention relates to collets and has particular reference to automatic machine tools as used for the machining of bar stock.
The automatic machines of this type are provided with mechanisms which in sequence open the collet advance the bar stock by the required amount and then close the collet onto the bar stock. Machining operations at the various work stations are made before the work piece is finally separated from the bar stock.
To ensure the bar stock is held securely during these machining operations it is essential that the collet clamps onto a substantial minimum length of bar stock. It is known to provide a warning mechanism to detect the end of the bar stock in the feed tube leading to the collet and to stop the machine when the bar stock is too short to be securely clamped. The short waste length of bar stock is then removed by hand.
It is also known to automatically check that the remaining bar stock is of sufficient length, and if not to eject it, so allowing the automatic machine to run continuously.
The bar stock is advanced through the collet by a feed tube located co-axially within the collet tube, the bar stock eventually making contact with a stock stop. The stock stop is movable between a first position and a second position. The first position determines the length of stock to be used but during feeding the stop is moved to a second position which requires the stock to be overfed by the feed tube. If the length of stock is such that it still remains in the jaws of the collet it is pushed back by the stock stop which moves to the first position before the collet is closed. If, however the stock is of insufficient length to reach the stock stop in the second position, then this short end falls clear. There are difficulties that arise from this use of the two position stock stop. The mechanism used to achieve the movable stop takes up room in the tooling area so restricting the tooling. The stock stop may also use a drive which might otherwise be used for a machine function. The reversal of the stop and bar stock imposes shock loads particularly with long bar stock feeding.